Volume 549, January 2013
|Number of page(s)||26|
|Section||Galactic structure, stellar clusters and populations|
|Published online||16 January 2013|
V. Evidence for a wide age distribution and a complex MDF
Lund ObservatoryDepartment of Astronomy and Theoretical
physics, PO Box
43, 221 00
2 Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210, USA
3 Palomar Observatory, Mail Stop 249-17, California Institute of Technology, Pasadena, CA 91125, USA
4 Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek, 2611 ACT, Australia
5 Departamento de Astronomia do IAG/USP, Universidade de São Paulo, Rua do Matão 1226, 05508-900 São Paulo, Brasil
6 INAF – Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
7 Department of Physics, Chungbuk National University, 361-763 Cheongju, Republic of Korea
8 Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
9 Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
10 Warsaw University Observatory, A1. Ujazdowskie 4, 00-478, Warszawa, Poland
11 University of Notre Dame, Notre Dame, IN 46556, USA
12 Institute for Information and Mathematical Sciences, Massey University, 1330 Auckland, New Zealand
13 Department of Earth and Space Science, Osaka University, 560-0043 Osaka, Japan
14 Solar-Terrestrial Environment Laboratory, Nagoya University, 464-8601 Nagoya, Japan
15 Mt. John Observatory, PO Box 56, 8770 Lake Tekapo, New Zealand
16 Department of Physics, Faculty of Science, Kyoto Sangyo University, 603-8555 Kyoto, Japan
Received: 31 October 2012
Accepted: 28 November 2012
Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. Combined with the sample of 26 stars from the previous papers in this series, we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) the metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H] = −1.9 to +0.6; (ii) the dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. A tentative interpretation is that there could be different stellar populations at interplay, each with a different scale height: the thin disk, the thick disk, and a bar population; (iii) the stars with [Fe/H] ≲ −0.1 are old with ages between 10 and 12 Gyr; (iv) the metal-rich stars with [Fe/H] ≳ −0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4−5 Gyr and a tail towards higher ages; (v) there are indications in the [α/Fe]−[Fe/H] abundance trends that the “knee” occurs around [Fe/H] = −0.3to −0.2, which is a slightly higher metallicity as compared to the “knee” for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and ...?), residing together in the central parts of the Galaxy, influenced by the Galactic bar.
Key words: gravitational lensing: micro / Galaxy: bulge / Galaxy: formation / Galaxy: evolution / stars: abundances
Based on data obtained with the European Southern Observatory telescopes (Proposal ID:s 87.B-0600, 88.B-0349, 89.B-0047, and 90.B-0204), the Magellan Clay telescope at the Las Campanas observatory, and the Keck I telescope at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration.
Tables 2–5 are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A147
© ESO, 2013
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